1. Field of the Invention
The present invention relates to a flexible display and a manufacturing method thereof. More particularly, the present invention relates to a flexible display applicable to an organic EL display, a liquid crystal display and the like which use a plastic film as a substrate, and a manufacturing method thereof.
2. Description of the Related Art
There has been a rapid increase in applications of a display device, such as an organic EL (electroluminescence) display and a liquid crystal display, to information device and the like. A flexible display using a plastic film as a substrate has recently attracted attention. Such a flexible display can be utilized not only for an ultraslim and light mobile device which can be housed while being rolled up and is easily carried around but also for a large-sized display.
However, since the plastic film has low rigidity and a low heat distortion temperature, heat distortion such as warping and expansion/contraction is likely to occur in manufacturing steps involving heat treatment. Thus, in a manufacturing method by which various elements are formed directly on the plastic film, conditions of the manufacturing steps involving heat treatment are limited, and it becomes difficult to perform high-accuracy alignment. Consequently, it may be impossible to manufacture an element substrate having desired characteristics.
In order to avoid the problem as described above, there is a method of manufacturing an element substrate for a liquid crystal device in the following manner (Patent Document 1 Japanese Patent Laid-Open Official Gazette No. 2003-131199)). Specifically, without having manufacturing conditions limited, a transparent electrode, a color filter layer and the like are formed on a heat-resistant and rigid glass substrate while being aligned with high accuracy. Thus, a transfer layer is formed. Thereafter, the transfer layer is transferred and formed on a plastic film.
Moreover, in order to obtain a display having excellent display characteristics, active drive including a drive transistor for each pixel is required. A flexible display requires flexible TFT elements which can follow bending. A low-temperature polysilicon TFT or an amorphous silicon TFT as a conventional drive transistor may not obtain sufficient reliability. Thus, as a drive transistor of the flexible display, an organic TFT using a flexible organic semiconductor layer, which can follow bending, as an active layer has attracted attention.
Patent Document 2 (Japanese Patent Laid-Open Official Gazette No. 2003-255857) describes a method of manufacturing an organic EL display in the following manner. Specifically, a gate electrode, a gate insulating film, an organic semiconductor layer and source and drain electrodes are sequentially formed on a substrate. Thereafter, an organic EL element is formed on an anode connected to the drain electrode.
Moreover, Patent Document 3 (Japanese Patent Laid-Open Official Gazette No. 2003-318195) describes a method of transferring an organic TFT, which is formed of a gate electrode, a gate insulating film, an organic semiconductor layer and source and drain, from a heat-resistant substrate to a surface substrate (a plastic substrate) after a separating layer is formed on the heat-resistant substrate and the organic TFT is formed thereon.
Meanwhile, as to an organic semiconductor layer and an organic EL layer, there is a problem that performance thereof is deteriorated by photolithography and etching steps, which involve an organic solvent, water, plasma, an electron beam, heat treatment or the like, and therefore the layers hardly function.
In Patent Documents 2 and 3 described above, it is required to pattern the source and drain and the like after the organic semiconductor layer is formed. Thus, deterioration in performance of the organic semiconductor layer in the photolithography step may become a problem.
As described above, a method of manufacturing a flexible display which uses a plastic film as a substrate and includes organic TFTs has not been sufficiently established. Therefore, there has been demanded a method of stably forming, with high yield, desired organic TFTs or organic EL elements on the plastic film.